The invention relates to a process for preparing a solution of polymers of conjugated dienes in vinylaromatic compounds, in particular styrene or substituted styrenes, and to a process for preparing impact-modified polymers of vinylaromatic compounds, in particular impact-modified polystyrene, by polymerization of this solution.
It is known that impact-modified polystyrene (high impact polystyrenexe2x80x94HIPS) can be prepared by catalytic polymerization of a solution comprising polymerized conjugated dienes in styrene or substituted styrenes.
DE-A 1770 392 describes the free-radical polymerization of a solution of from 1 to 14% by weight of a rubber-like polymer in a polymerizable vinylaromatic compound in a plurality of stages to a solids content of more than 60% by weight. This finally gives a solid dispersion of rubber particles in a polystyrene base. Here, polystyrene chains are also grafted onto the rubber to varying degrees.
The solution used in the polymerization is obtained by dissolving the rubber-like diene polymer in the vinylaromatic monomer. This procedure is cumbersome, since it requires firstly the preparation of the diene polymer, the work-up and isolation thereof and subsequently dissolution in the vinylaromatic monomer. Preparation of the impact-modified vinylaromatic polymer by polymerization of the monomers in one step would be desirable.
EP-A 59231 describes the anionic polymerization of butadiene in styrene as solvent in the presence of alkyllithium as catalyst, giving a styrene-butadiene rubber having a low styrene content. After the polymerization has been stopped, the remaining monomeric butadiene is separated off and the styrene is, if desired after addition of further amounts of this monomer, copolymerized to give impact-modified polystyrene. Due to the relationship between the copolymerization parameters of butadiene and styrene in this system and the high styrene concentration, a considerable proportion of styrene is built into the polymer even at a conversion of only 30% of the butadiene, which results in an undesirable increase in the glass transition temperature of the rubber phase in the finished product. Although the reaction can be stopped before complete conversion of the butadiene, the polybutadiene formed then has to be purified by precipitation or the solvent together with other volatile substances, in particular monomeric butadiene, has to be distilled off.
WO 98/07765 describes the anionic polymerization or copolymerization of butadiene in styrene solution in the presence of catalysts comprising mixed alkali metal alkyls. Here too, a copolymer having a high styrene content is obtained.
It is an object of the present invention to propose a process for preparing a solution of high molecular weight polymers of conjugated dienes in a vinylaromatic hydrocarbon, which solution can be used without complicated isolation or separation steps to prepare impact-modified polymers of the vinylaromatic hydrocarbon which comprise virtually no monomeric diene and a polydiene phase having a low content of units of polymerized vinylaromatic compounds.
The present invention starts out from a process for preparing a solution of high molecular weight polymers of conjugated dienes in a vinylaromatic hydrocarbon by anionic polymerization of the diene in the presence of an alkyllithium as catalyst.
In the process of the present invention, the diene is firstly polymerized in solution in an aromatic hydrocarbon which is free of ethylenic double bonds to a molecular weight Mw below about 105, a trialkylaluminum is then added in a molar excess relative to the alkyllithium, the solution obtained is diluted with the vinylaromatic compound and the polydiene is then coupled with a polyepoxide or an alkyl (meth)acrylate to give a high molecular weight polymer.
The present invention also provides a process for preparing impact-modified polymers of vinylaromatic compounds, in particular impact-modified polystyrene, which comprises polymerizing a solution obtained by the above-described process, if desired after addition of further amounts of vinylaromatic compound, in a manner known per se.
The anionic diene polymerization according to the present invention forms a living polymer, i.e. a polymer whose chain formation is ended by consumption of the monomer and not by a termination or deactivation reaction. This means that the polymerization continues at the end of the chain when further amounts of monomer are added.
The polymerization according to the present invention of a conjugated diene is carried out in solution in an aromatic hydrocarbon, for example in toluene, xylene or ethyl benzene. The amount of solvent should be sufficient for the viscosity of the resulting polydiene solution to remain low enough to ensure good mixing of the reaction solution. On the other hand, the amount of inert solvent should be kept as small as possible in order to keep the energy requirement for separating it from the end product low or even to avoid the separation completely. In general, the amount is chosen so that a 20-40% strength, preferably 25-35% strength, polydiene solution is obtained.
As conjugated dienes, preference is given to those having 4-7 carbon atoms, e.g. butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene and 1,3-hexadiene, in particular butadiene and isoprene.
The process of the present invention makes it possible to obtain a polymer which has a molecular weight which is not too high, approximately in the range below 105, preferably from 104 to 105, in the first stage of the diene polymerization by appropriate introduction of the catalyst, so that the solution of the polymer has a comparatively low viscosity, since this depends exponentially on the molecular weight of the dissolved material. In this first polymerization stage, it is particularly advantageous to use a mixture of alkyllithium and trialkylaluminum as catalyst, with the molar amount of the trialkylaluminum being smaller than that of the alkyllithium, calculated as mol of Al/mol of Li. It is preferably 0.1-0.9:1. The trialkylaluminum acts as a retarder which moderates the catalytic activity of the alkyllithium but does not prevent the polymerization of the diene. A further effect of the catalyst mixture is that the polymer chains formed have a lower tendency to associate formation and thus form solutions having a lower viscosity.
The alkyllithium and trialkylaluminum compounds used in the process of the present invention preferably contain alkyl groups having from 3 to 6 carbon atoms. These may be linear or branched; in the case of the aluminum compounds, branched alkyl radicals are preferred. Particular preference is given to C4-alkyl radicals.
In the next stage of the polymer preparation, the molecular weight is increased by coupling the polydiene anions with a coupling agent which is able to react at least bifunctionally with the polymer chains. As coupling agents, use is made of polyepoxides or alkyl (meth)acrylates. Firstly, a molar excess of Al over Li is established by addition of trialkylaluminum. This reduces the catalytic activity to such an extent that it is no longer sufficient to polymerize vinylaromatic hydrocarbons, in particular styrene. The excess is preferably in the range from 1.2 to 1.7 mol of Al per mol of Li. Monomeric vinylaromatic is then added to give a more dilute solution whose viscosity allows a further molecular weight increase by coupling. The coupling agent should be at least bifunctional, i.e. contain at least two groups capable of reacting with the polymer anion. (Meth)acrylates, i.e. acrylates or methacrylates, can react both via the ester groups and via the double bonds. As has been found, a triple coupling reaction on a (meth)acrylic ester group is also possible. Examples of suitable coupling agents are ethylene glycol diglycidyl ester, 1,4-butanediol diglycidyl ester, methyl methacrylate, n-butyl acrylate, ethyl methacrylate, isopropyl acrylate and 1,4-butanediol diacrylate. Use is generally made of alkyl (meth)acrylates having from 1 to 6 carbon atoms in the alkyl group. Although the trialkylaluminum lowers the activity sufficiently to prevent polymerization of vinylaromatics, coupling with the more active coupling agents mentioned is still possible. The coupling reaction is preferably carried out at elevated temperature, in particular in the range 30-70xc2x0 C.
As vinylaromatic compounds, preference is given to using low molecular weight, i.e. monocyclic, aromatics, in particular styrene or substituted styrenes, such as xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene and also 1,1-diphenylethylene; unsubstituted styrene is particularly preferred.
The rubber solution obtained from the anionic diene polymerization and the subsequent coupling reaction can be used directly or after dilution with further vinylaromatic compound to prepare an impact-modified vinylaromatic polymer, in particular impact-modified polystyrene. The polymerization of the vinylaromatic can be carried out anionically or by a free-radical mechanism.
Suitable free-radical initiators are peroxides, for example diacyl peroxides, dialkyl peroxides, diaryl peroxides, peroxyesters, peroxydicarbonates, peroxyketals, peroxosulfates or hydroperoxides, or azo compounds. Preference is given to using dibenzoyl peroxide, 1,1-di-tert-butylperoxocyclohexane, dicumyl peroxide, dilauryl peroxide and azobisisobutyronitrile.
As auxiliaries, it is possible to add molecular weight regulators such as dimeric xcex1-methylstyrene, mercaptans such as n-dodecyl mercaptan or tert-dodecyl mercaptan, chain branching agents, stabilizers or mold release agents.
The polymerization of the matrix can be carried out in bulk or in solution. The polymerization is generally carried out at from 50 to 200xc2x0 C., preferably from 90 to 150xc2x0 C., in the case of free-radical polymerization, and at from 20 to 180xc2x0 C., preferably from 30 to 80xc2x0 C., in the case of anionic polymerization. The reaction can be carried out isothermally or adiabatically.